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Super BN

External Components

Principles of Carburetion

For Regulator Portion

Because the Super BN is a watercraft carburetor, it cannot vent

its fuel chamber to the atmosphere like a carb with a loat system; this

would cause fuel leakage into the hull in the case of a roll over. The Super

BN’s fuel chamber is contained by the regulator diaphragm, with fuel on

one side and atmospheric pressure on the other.

As the fuel in the chamber is drawn in by engine suction, the

diaphragm moves in response to the depletion of fuel and the effect of

atmospheric pressure pushing against it. The diaphragm contacts and

moves the needle valve arm allowing fresh fuel to enter the fuel chamber.

As fuel enters the fuel chamber it is being pushed in under pressure from

atmospheric pressure. When the fuel chamber is once again illed, the

diaphragm relieves the pressure in the arm and the needle valve closes.

In this way, the diaphragm mechanism precisely regulates the amount of

fuel in the fuel chamber.

Once again, the regulator portion of the Super BN primarily

affects tuning at throttle openings of 1/4 or less in response to the

engines high manifold pressure. The regulator portion of the BN

engine coniguration. The terms most commonly used in this

adjustment is “Pop-Off” pressure.

Pop-off is a relative term,, it refers to the pressurization of the

needle valve with compressed air, through the fuel inlet, and nothing the

pressure necessary to open the needle valve or “pop” it off its

seat. The higher the pressure that is required indicates a higher pop-


For Regulator Portion

The Super BN starts performing like a conventional carbs as the

throttle is opened through and past 1/4 in that is uses replaceable jets and

adjuster screws to make changes in calibration. At or beyond 1/4 throttle

opening, the engine replaces manifold pressure with air velocity.

What is the difference between manifold pressure and air

velocity? Manifold pressure is the suction generated by the engine against

a very restrictive mechanism within the carb: The closed or nearly closed

throttle plate. As the throttle is opened, the carb transition from being

controlled by the regulator portion to responding as a normal carb does.

High air velocity, created by the engine as it revs up, appears as

a strong suction (or low pressure) within the carb body. Fuel low through

the carb is caused by the differential in pressure between the positive

atmospheric pressure on the diaphragm and the suction within the carb

body. Because air velocity creates such a strong suction, pop-off pres-

sure has virtually no affect on calibration past about 1/4 throttle.


Outer Venturi

Inner Venturi

Primer Fitting

High Speed Adjuster

Pulse Nipple

Fuel Inlet

Fuel Return

Fuel Pump Assembly

Idle Stop Screw

Low Speed Adjuster

Regular Diaphragm

Arm and Arm Spring

High and Low Speed Jets

Needle Valve Assembly